The present invention relates to high-pressure equipment and more particularly to a method of manufacturing multilayer convex collared covers and bottoms for pressure vessels.
The present invention can be used in designing the high-pressure equipment in, say, chemical and petrochemical industries.
Moreover, the present invention can be most efficiently used in building large pressure vessels for internal pressures above 100 kgf/cm.sup.2.
Initially, multilayer pressure vessels consisted in part of several layers, namely, only their cylindrical portion consisted of such several layers, while the end portions, i.e. the bottom and the cover, were of a solid construction, e.g. forged.
Such forged covers and bottoms are expensive and their manufacture involves a large amount of labour. Their production calls for a large amount of machining and results in heavy waste of metal. Production of forged covers and bottoms additionally requires complicated metallurgical and forging equipment. Lastly, it is difficult to attain high mechanical properties of metal in thick forgings.
All this taken together tends to raise the cost of pressure vessels and impairs their reliability.
The solid bottom is primarily made by hot stamping it from thick sheets on powerful presses, whereas, the solid covers are made from forgings.
Small-diameter vessels are provided with flat covers whose thickness may reach high limits depending on pressure. However, the large-diameter vessels are made with convex collared covers, and such covers are usually of two parts, namely, a flange, in the form of a solid ring and a solid hemisphere, welded to the flange. The hemisphere is hot stamped from thick metal sheets.
Known in the previous art is a method of making flanged covers from a single sheet (a forging). This method utilizes stamping combined with clamping.
However, the production of a solid bottom from a thick sheet and a cover forging such material is extremely labourous consuming and involves the use of complicated equipment.
Therefore, the manufacture of covers and bottoms by forging solid material for large pressure vessels is quite difficult.
This has posed a problem of making multilayer flanged covers and bottoms for pressure vessels.
There is known a method of making multilayer bottoms for pressure vessels wherein the bottom is made from individual sheet blanks. In this method, the first stage consists in stamping the first inner layer of the bottom in a known manner while the metal is hot after which this layer is used as a punch holder and the second, third, etc. layers are stamped until the required thickness of the bottom is attained.
After stamping each subsequent layer, either the die or its inserted rings are changed. The size of each subsequent ring of the die should correspond to the diameter of the bottom layer. This method does not require complex stamping equipment and is adaptable in making the bottom from thin sheets.
The utilization of this method, however, in making large-diameter thick bottoms requires the use of a set of dies o replaceable die rings.
In the course of heating and stamping, each sheet becomes covered with a layer of scale which remains between the layers, thus increasing the interlayer clearances and impairing the reliability of the bottom. Besides, the bottom manufacturing process is complicated by a multitude of successive stamping operations.
Known in the art is a method of manufacturing multilayer convex collared covers and bottom for pressure vessels which consists of stamping the cover or bottom from a pack of circular sheet blanks in a known manner while the metal is cold. The pack is assembled by placing one blank on another to form a truncated cone. The diameter of the upper smaller blank is equal to the developed length of the inner layer of the bottom or cover. The pack in the form of a truncated cone is placed on a die and clamped by a hold-down device. In the course of stamping, the pack is drawn to the shape of the punch.
This method produce makes it possible to the bottom or cover of a higher quality than previously described methods, decreases the interlayer clearances and reduces the amount of labour required.
However, the necessity for using the hold-down device in the course of stamping, increases the force of the press by 50% as compared with the previously described method.
In manufacturing the large-diameter thick multilayer steel bottoms and covers, stamping must be followed by heat treatment for relieving the internal stresses and ensuring the preset plastic properties of metal.
Heat treatment increases the interlayer clearances; as a result, air penetrates into these clearances and scale is formed therein, thereby worsening the finished product.
Furthermore stamping of covers and bottoms for large-diameter thick pressure vessels according to the above known method (where the metal being stamped is cold) involves the employment of powerful presses since the yield point of metal in a cold state is much higher than that of hot metal.